TILLMAN, RODERICK W.
Consulting Geologist, Tulsa, OK

Abstract: Estuarine Sandstone Reservoirs

Estuarine reservoirs are commonly overlooked or misinterpreted in environment of deposition reconstruction of paleogeography. Two major types of estuarine reservoirs occur. One type forms as a result of progradation of tidally dominated or tidally influenced deltas and the other results from backfilling of incised valleys or bays. I will focus on the estuaries that are backfilled incised valleys or bays.

Estuary deposits normally contain heterolithic deposits which include current, tide and some cases wave deposited sediments. Broad open estuaries (bays) may contain significant amounts of wave influenced deposits while narrow estuaries are more likely to be tidal and/or fluvially dominated. Recognition of tidal influences is a key to identifying most estuaries. Ravinement and parasequence or sequence boundaries commonly bracket or at least form one contact of estuarine deposits.

Estuarine reservoirs include tidal sand ridges, tidal channel fill, accretion-bars, bayhead deltas, and tidally influenced fluvial channel fill. Wave dominated features, including spits and bars may occur within or at the mouth of estuaries. Examples of estuarine deposits in the Western Interior Seaway of the U.S include the Muddy Sandstone, Frontier and Shannon Formations. Many of the estuarine deposits in the Muddy are confined to relatively narrow incised valleys which were backfilled during the final stages of a Lowstand Sequence or in the initial stages of a Transgressive Sequence Tract. Tidal accretion bars form significant fill in the Muddy. In the Frontier bayhead deltas and other estuarine deposits are commonly bracketed by marine sequence boundaries at the base and flooding surfaces at the top. These abrupt relative sea level changes are attributed to local tectonics during Frontier time.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas